Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared (IR) radiaton and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or particle coagulation of a thermoplastic polymer latex.
Negative working thermal sensitive printing plate precursors, comprising thermoplastic polymer particles of which the imaging mechanism is based on coalescence or coagulation of the polymer particles have been disclosed in several applications. For example EP-As 770 494, 770 495, 770 496 and 770 497 disclose printing plate precursors comprising thermoplastic particles which, after exposure, are processed on press by supplying ink and/or fountain solution. EP-A 1 342 568 and WO2006/037716 disclose a method of making a lithographic printing plate wherein a precursor comprising thermoplastic particles is, after exposure, processed in a gum solution. EP-As 1 614 539 and 1 614 540 disclose a method of making a lithographic printing plate wherein a precursor comprising thermoplastic particles is, after exposure, processed in an alkaline solution.
EP-As 1 736 312 and 1 910 082 disclose lithographic printing plate precursors comprising thermoplastic particles and an IR-dye, said dye being capable of forming a print out image upon exposure to IR radiation. The visible print out image is formed by a chemical transformation of the IR-dye upon exposure to IR radiation. Precursors capable of forming a print out image upon IR exposure are very well suited to be used in an on-press processing set-up wherein the exposed precursor is mounted on the press and the non-image areas are subsequently removed by applying ink and/or fountain to the mounted precursor. The formation of a print out image enables a visible inspection of the non-processed precursor before mounting it on the press.
EP-As 1 859 935 and 1 859 936 disclose a lithographic printing plate precursor comprising thermoplastic particles having an average particle size between 10 and 40 nm and wherein the amount of IR-dye is adjusted as function of the particle size of the polymer particles.
EP-A 08 105 354.8 (filed on 2008 Sep. 16) discloses a lithographic printing plate precursor comprising a coating provided on a support having a hydrophilic surface, the coating containing thermoplastic polymer particles and an infrared radiation absorbing dye characterized in that the dye contains a substituent selected from bromo and iodo.
Various IR-dyes have been disclosed which can be used in printing plate precursors. Most of them have as central structure element a 5 or 6 membered ring as shown below.

The synthesis of IR-dyes having such a central 5 or 6 ring is relatively easy, several of them are commercially available, and provide sufficient lithographic properties to precursors containing them.
However, it has been observed that a disadvantage of precursors comprising such IR-dyes may be a poor stability when stored under daylight conditions. It seems that such IR-dyes may be oxidised when the precursors are stored under daylight. This instability may result in a loss of sensitivity, a worsening of the clean-out behaviour, i.e. removal of the non-image areas during development, and a change of colour of the precursor. These phenomena become more pronounced under prolonged storage under daylight conditions. Exposure to daylight may occur, for example, for the uppermost precursor of a stack of precursors loaded in a platesetter. It has been observed that, when the platemaking process is resumed, for example after a couple of days, the lithographic properties of such an uppermost precursor of a stack of precursors may have changed compared with the other precursors of the stack.